Clinical Note

Pituitary dwarfism is the failure of growth that results from lack of GH during childhood. Pituitary dwarfs typically are of normal weight and length at birth and grow rapidly and nearly normally during early infancy. Before the end of the first year, however, growth is noticeably below the normal rate and continues slowly for many years. Left untreated, they may reach heights of around 4 feet. Typically, the pituitary dwarf retains a juvenile appearance because of the retention of "baby fat'' and the disproportionately small size of maxillary and mandibular bones. Pituitary dwarf-ism is not a single entity and may encompass a range of defects. The deficiency in GH may be accompanied by deficiencies in several or all other anterior pituitary hormones (panhypopituitarism) as might result from defects in pituitary development (see Chapter 38). Alternatively, traumatic injury to the pituitary gland or a tumor that either destroys pituitary cells themselves or their connections to the hypothalamus might also interfere with normal pituitary function. These individuals do not mature sexually and suffer from inadequacies of thyroid and adrenal glands as well. The lack of GH might also be an isolated inherited defect, with no abnormalities in other pituitary hormones. Aside from their diminutive height, such individuals are normal in all respects and can reproduce normally. Causes of isolated GH deficiency are multiple, and include derangements in synthesis, secretion, and end-organ responsiveness.

Overproduction of GH may occur either as a result of some derangement in mechanisms that control secretion by normal pituitary cells or from tumor cells that secrete GH autonomously. Overproduction of GH during childhood results in gigantism, in which an adult height in excess of 8 feet has occasionally been reported. Overproduction of GH during adulthood, after the growth plates of long bones have fused (see below), produces growth only by stimulation of responsive osteoblastic progenitor cells in the periosteum. There is thickening of the cranium and mandible, as well as enlargement of some facial bones and bones in the hands and feet. Growth and deformities in these acral parts give rise to the name acromegaly to describe this condition. Persistence of responsive cartilage progenitor cells in the costochrondral junctions leads to elongation of the ribs to give a typical barrel-chested appearance. Thickening of the skin and disproportionate growth of some soft tissues, including spleen and liver, are also observed in acromegalic patients.

normal growth. Attainment of adult size is absolutely dependent on GH; in its absence, growth is severely limited, and when it is present in excess, growth is excessive.

When thinking about giants and dwarfs, it is important to keep in mind the limitations of GH action. The pediatric literature makes frequent use of the term genetic potential in discussions of diagnosis and treatment of disorders of growth. Predictions of how tall a child will be as an adult are usually based on the average of parental height plus 2.5 inches for boys or minus 2.5 inches for girls. We can think of GH as the facilitator of expression of genetic potential for growth rather than as the primary determinant. The entire range over which GH can influence adult stature is only about ^30% of genetic potential. A person destined by his genetic makeup to attain a final height of 6 feet will attain a height of about 4 feet even in the absence of GH and is unlikely to exceed 8 feet in height even with massive overproduction of GH from birth. We do not understand what determines the genetic potential for growth; but it is clear that, although both arise from a single cell, a hypopituitary elephant is enormously larger than a giant mouse. Within the same species, something other than aberrations in GH secretion accounts for the large differences in size of miniature and standard poodles or Chihuahuas and Great Danes.

Synthesis, Secretion, and Metabolism

Although the anterior pituitary gland produces at least six hormones, more than one-third of its cells synthesize and secrete GH. Of the GH produced by somatotropes, 90% is comprised of 191 amino acids and has a molecular weight of about 22,000; the remaining 10% (called 20K GH) has a molecular weight of 20,000 and lacks the 15-amino-acid sequence corresponding to residues 32 to 46. Both forms are products of the same gene and result from alternate splicing of the RNA transcript. Both forms of hormone are secreted and have

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